A:
Brimstone is mentioned 15 times in the Bible (KJV). The Hebrew word gophriyth is used
7 times
for "brimstone" in the Old Testament, and the Greek word theion is used 7 times in the New Testament.
(Theiodes is used in once in Rev 9:17.)
Most agree that "brimstone" is the same as what we call "sulfur" or "sulphur"
today. (Some modern translations [i.e. NIV, ESV, NLT] use "sulfur" instead of
"brimstone.")

Sulfur is a yellow colored mineral that is
highly inflammable (easily set on fire). Many things
we use today contain traces of sulfur, such as: gunpowder, car
batteries, tires, medicines, matches, paint, and more.... When sulfur /
brimstone is ignited, it burns in a dark blue color, and emits a terrible smell,
which smells like rotten eggs. (Back in my younger days, someone I knew burned
sulfur, and I still remember this smell.)
Breathing in the fumes of burning sulfur in high quantities can be toxic. Sulfur
can also be found in the lava of volcanoes, in hot springs, and the smell can
sometimes be detected after a lightning storm.

***Note: There is a short video floating around online of a brimstone /
sulfur fire in Wyoming that is fascinating and mesmerizing to watch. It
looks like a lake on fire. (Search: "sulfur fire in Wyoming")

Perhaps the most famous mention of "brimstone"
in the Bible is in (Gen 19:1-26), when God "rained brimstone and fire....out
of heaven," and destroyed Sodom and Gomorrah (Gen 19:24) because of their
wickedness
(homosexuality / sodomy) being the primary sin)
(also see: Lk 17:29). How this occurred is up for
debate. (***Note: Some ancient sources say lightning.)

In addition to this incident, brimstone is connected with God's
judgment in other places too (Deut 29:23)(Ps 11:6)(Isa 30:33)(Isa 34:9)(Ezek
38:22)(Rev 9:17-18). The ultimate judgment is found in the book of Revelation, where we
find the "lake of fire," which "burns with fire and brimstone" (Rev
14:10)(Rev 19:20)(Rev 20:10)(Rev 21:8). This
will be the eternal final destination for the devil, his angels, and all
humans who followed him (those who did not accept Jesus as their Lord and
Savior: Rev 21:6-8).

***Note: Today, it is believed by many scientists that the remains of Sodom
and Gomorrah lie near the south end of the Dead Sea. This area has very
high concentrations of brimstone.